1. Field of the Invention
This invention relates to an Oldham coupling for an electric motor, a manufacturing process thereof, a coupling process of shafts by the Oldham coupling, and an electric motor including the Oldham coupling.
2. Description of the Related Art
An Oldham coupling for transmitting rotational power between two rotational members whose rotational axes are eccentric is known (see JP-A-63-289324, JP-A-3-79814, JP-B-3602404, JP-2007-285356, and JP-B-4161365). The Oldham coupling includes a pair of coupling parts which are connected to rotational shafts, respectively. The Oldham coupling also includes an intermediate part which is slidably fitted to the coupling parts. The coupling parts are fitted to a fitting portion in the form of a protrusion or a groove on both end surfaces of the intermediate part, respectively. The two fitting portions extend perpendicular to each other. Since the coupling parts are slidably fitted to the intermediate part, respectively, in the case where the two rotational shafts are eccentric to each other, the intermediate part moves relative to each coupling part so as to compensate the amount of eccentricity. With such a feature of the Oldham coupling, it is advantageous that greater allowance in the amount of eccentricity can be ensured.
In general, two coupling parts of an Oldham coupling are fitted on fitting portions of an intermediate part extending perpendicular to each other, and therefore, a positional relationship between the coupling parts about a rotational axis is predetermined. This is advantageous to, for example, a synchronous electric motor in which a relationship between a reference position of a rotational shaft of the electric motor and a reference position of a rotational shaft of a detector needs to be determined. When an Oldham coupling is used to connect an electric motor and a detector to each other, attaching/detaching one of the coupling parts to/from the rotational shaft can be carried out independently of the other of the coupling parts. Therefore, attachment and exchange processes can be advantageously facilitated.
However, in some cases, it may be inappropriate to apply the Oldham coupling to a synchronous electric motor because of the structural characters of the Oldham coupling. The fitting portions of the intermediate part generally have an elongated shape extending through the center of the intermediate part. Such fitting portions have a symmetric shape in plan view relative to a given line extending through the center of the intermediate part. Therefore, the coupling part and the intermediate part can be mechanically fitted to each other when the coupling part is rotated relative to the intermediate part by every 180 degrees. On the other hand, it is necessary for a synchronous electric motor to precisely detect rotational angular positions of the electric motor in order to supply windings with electric current at an appropriate phase. Accordingly, it is necessary to determine a positional relationship between the electric motor and the detector about a rotational axis, in which their referential positions correspond to each other. However, it is possible to assemble the Oldham coupling with only one of the two coupling parts rotated by 180 degrees from its reference position. If this is the case, rotational angular positions may be erroneously detected by 180 degrees for some electric motors, resulting in electric current at an inappropriate phase being supplied to windings. In order to avoid this from occurring, an additional process is required to check and adjust, if necessary, the phases of the electric motor and the detector when the Oldham coupling is used for this purpose.
According to the present invention, an Oldham coupling can be provided in which it is ensured that the coupling parts are connected to each other in a desired positional relationship by allowing the Oldham coupling to determine a positional relationship between the coupling parts. According to the present invention, a manufacturing process for manufacturing such an Oldham coupling can also be provided. Further, according to the present invention, a coupling process for coupling a rotational shaft of an electric motor to a rotational shaft of a detector by way of such an Oldham coupling can be provided.